1. Field of the Invention
The present invention relates to a method of joining or reflow soldering metallic members by use of electric current having an AC waveform.
2. Description of the Related Arts
Reference is made to FIGS. 11A-11C depicting by way of example a fusing process which is one method of joining metallic members together. This process includes electrically and physically joining an insulated wire 2 and a terminal plate 4 made of copper or copper alloy.
First, as shown in FIG. 11A, workpieces W consist of the terminal 4 and the insulated wire 2 placed inside a hooked portion or a bent portion 4a of the terminal 4. The workpieces W are interposed between a pair of (e.g., upper and lower) electrodes 6 and 8 in such a manner that the lower electrode 8 provides a support for the underside of the terminal hooked portion 4a at a given position while simultaneously the upper electrode 6 is abutted against the top face of the terminal hooked portion 4a so that the top face is pressed down by a pressure device (not shown) with a predetermined pressing force F. At the same time, a predetermined voltage is applied across the two electrodes 6 and 8 by a power supply apparatus (not shown).
Then, first, an electric current I flows across the two electrodes 6 and 8 through the terminal hooked portion 4a acting as a current path, with the result that Joule heat is generated in the terminal hooked portion 4a. Consequently, as shown in FIG. 11B, an insulator 2a of the insulated wire 2 is melted by the Joule heat and is removed from a conductor 2b. 
After removal of the insulator 2a, as shown in FIG. 11C, the current I flows across the two electrodes 6 and 8 by way of the conductor (typically, copper) 2b of the insulated wire 2. A pressing force F is continuously applied across the two electrodes 6 and 8 during the current-supplying period, and hence the Joule heat cooperates with the pressing force F to jointly pressure-join and crush the hooked portion 4a and the insulated wire conductor 2b together for caulking. This allows the insulated wire 2 and the terminal 4 to be firmly electrically and physically joined together. Due to an extremely small resistance of the conductor 2b of the insulated wire 2 and the terminal 4, no nugget (weld joint) will be formed therebetween.
FIG. 12 shows the circuit configuration of a single-phase AC power supply apparatus which has hitherto been used for the above fusing process. FIG. 13 shows waveforms of a voltage and current appearing in this power supply apparatus.
In this power supply apparatus, a single-phase AC voltage V of a commercial frequency input to input terminals 100 and 102 are fed to a primary coil of a step-down transformer 108 via a contactor composed of a pair of thyristors 104 and 106. An AC induced electromotive force (secondary voltage) generated in the secondary coil of the transformer 108 is applied via the secondary conductor and the electrodes 6 and 8 are applied to the workpieces W (2, 4) so that a secondary current i2 having a larger current value than a primary current i1flows as the fusing current I through the secondary circuit.
Although the magnitude (effective value) of the fusing current I (i2) is determined by the conduction angle, it may also be said that the magnitude depends on the firing angle due to a substantially constant relationship between the firing angle and the conduction angle. In this power supply apparatus, a control unit 110 controls the firing angle (firing timing) xcex8 of the thyristors 104 and 106 by way of a firing circuit 112, to thereby control the effective value of the fusing current I (i2).
The control unit 110 accepts a start signal from an external device (not shown) such as a conveyor robot associated with the workpieces W (2, 4), causes the pressure device to commence its pressing operation, executes the above thyristor firing control over a preset current-supplying time after the elapse of a predetermined time (squeeze time), and upon the termination of the current-supplying time, allows the pressure device to release the pressure after the elapse of a predetermined time (hold time).
In the above fusing process, the workpieces W (2, 4) are thermocompressively joined by the Joule heat and the pressing force. However, excessive thermocompressive joining may remarkably impair the shape of the joined parts or may possibly induce breakage thereof or damage thereto. A monitoring device is thus provided for detecting the displacement of the upper electrode 6 in the pressing direction to monitor the degree of thermocompressive joining of the workpieces W (2, 4). Once the displacement of the upper electrode 6 reaches a predetermined value (limit value), the monitoring device issues a current-halting signal, in response to which the control unit 110 brings the supply of current to a halt.
However, since the prior art employs the above thyristor firing control method, its current control ability may be lost after firing or turning on the thyristors 104 and 106 as shown in FIG. 14, so that the supply of current cannot be halted until the termination of that half cycle (10 ms in case of 50 Hz). For the duration, the thermocompressive joining of the workpieces W (2, 4) may proceed excessively and render the time lag in halting the supply of current fatal.
The present invention was conceived in view of the above problems. It is therefore an object of the present invention to provide a metallic members joining method and reflow soldering method in which supply of current (regular supply of current) is ceased at once even in mid course of the AC waveform when situations to halt the supply of current occur prior to the termination of the set current-supplying time in the joining or reflow soldering of the metallic members using the AC waveform current, thereby enabling good quality workpieces work qualities to be obtained.
Another object of the present invention is to provide a metallic members joining method and reflow soldering method in which the regular supply of current is interrupted the instant that a halt signal is received during the supply of current and in which a small electric current of such a magnitude as not to substantially affect the joining and soldering flows is provided depending on the state of supply of current (elapsed time and polarity) upon the receipt of the halt signal, thereby preventing the transformer from undergoing the polarized magnetism or magnetic saturation, to consequently assure the protection or reliability of the power supply unit.
In order to achieve the above objects, according to a first aspect of the present invention there is provided a metallic members joining method in which a pair of electrodes are pressed against metallic members while simultaneously an AC waveform current flows through the pair of electrodes, the metallic members being joined together by making use of Joule heat generated in the metallic members. The method comprises: converting an AC voltage of a commercial frequency by a rectifying circuit into a DC voltage; converting the DC voltage output from the rectifying circuit by an inverter into a pulsed voltage of a high frequency; applying the high-frequency pulsed voltage output from the inverter across a transformer and via the pair of electrodes to the metallic members without rectification on the secondary side of the transformer; segmenting a current-supplying time for a single joining into a plurality of unit current-supplying periods and switching the inverter at the high frequency so as to allow an AC waveform current having a set current value to flow through the secondary side of the transformer with one polarity in each odd-numbered unit current-supplying period but with the other polarity in each even-numbered unit current-supplying period; and when predetermined phenomena or predetermined conditions to halt the regular supply of current occur or are satisfied during the current-supplying time, bringing switching operations of the inverter to a halt to interrupt a regular supply of current.
According to a second aspect of the present invention there is provided a reflow soldering method in which with a solder intervening between sites to be soldered of metallic members, a resistance heating heater tip is abutted against the metallic members while simultaneously an AC waveform current flows through the heater tip, the sites to be soldered of the metallic members being soldered by making use of Joule heat generated in the heater tip. The method comprises: converting an AC voltage of a commercial frequency by a rectifying circuit into a DC voltage; converting the DC voltage output from the rectifying circuit by an inverter into a pulsed voltage of a high frequency; applying the high-frequency pulsed voltage output from the inverter across a transformer to the heater tip without rectification on the secondary side of the transformer; segmenting a current-supplying time for a single soldering into a plurality of unit current-supplying periods and switching the inverter at the high frequency so as to allow an AC waveform current having a set current value to flow through the secondary side of the transformer with one polarity in each odd-numbered unit current-supplying period but with the other polarity in each even-numbered unit current-supplying period; and the instant that predetermined phenomena or predetermined conditions to halt the regular supply of current occur or are satisfied during the current-supplying time, bringing switching operations of the inverter to a halt to interrupt a regular supply of current.
In the present invention, the inverter disposed on the primary side of the transformer is switching controlled at a high frequency and the polarity of the inverter, i.e., the polarity of the electric current is inverted at a predetermined cycle so that an AC waveform electric current flows through the secondary side and that the metallic members are joined or reflow soldered by use of this AC waveform electric current. Since the inverter performs its switching operations at the above high frequency, an instantaneous halt of the supply of current can be achieved with a time lag of one cycle at most. Thus, even when the secondary current is in mid course of the AC waveform, it is possible to interrupt and terminate the regular supply of current at any point in time.
Preferably, the methods of the present invention may further comprise the step of providing a switching control of the inverter so as to allow an electric current for erasing residual magnetism to flow through the secondary side of the transformer depending on the elapsed time and polarity in the unit current-supplying period upon the halt of supply of current. In this event, it is preferred that the electric current for erasing residual magnetism have such a current value as not to substantially affect joining of the metallic members.